Probing porous low-dielectric constant thin films using positronium annihilation lifetime spectroscopy

Depth-profiled positronium annihilation lifetime spectroscopy (PALS), as a unique, non-destructive technique, has been used to probe important issues related to the novel porous low-dielectric constant (k) thin films and their integration with Cu. It is sensitive to all pores (both interconnected and closed) in the size range from 0.3 nm to 300 nm, even in films buried under a top overlayer. Methodologies to explore the pore characteristics (size, distribution, and interconnectivity) in thin (sub-micron), porous low- k films using PALS have been developed along with a model calibration that relates the positronium annihilation lifetime with pore size. PALS has been compared with other techniques in an ongoing round-robin characterization of pores in various low-k films. The pore size results to date are found to be highly consistent.; The first application of PALS in studies related to Cu/low-k integration has been explored. PALS is found to be sensitive to metal overlayer diffusion into the porous structure of low-k films. The technique is useful for investigating the continuity and thermal stability of thin barrier layers designed to prevent Cu diffusion into the porous film. The minimum required barrier thickness on blanket samples has been determined and procedures to test barrier performance on realistic samples with trenches or vias have been proposed. The effects of pore size and surface roughness of the porous films on the minimum barrier thickness are studied. PALS has also been used to investigate the structural change of porous low- k films during the processing steps required for their integration with Cu, such as plasma exposure-induced film densification.